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144 results on '"Francoeur S"'

1. Probing Hyperbolic and Surface Phonon-Polaritons in 2D materials using Raman Spectroscopy

Author

Bergeron, A., Gradziel, C., Leonelli, R., and Francoeur, S.

Subjects
Condensed Matter - Materials Science
Abstract

The hyperbolic dispersion relation of phonon-polaritons (PhPol) provides high-momentum states, highly directional propagation, subdiffractional confinement, large optical density of states, and enhanced light-matter interactions. In this work, we use Raman spectroscopy in the convenient backscattering configuration to probe PhPol in GaSe, a 2D material presenting two hyperbolic regions separated by a \textit{double} reststrahlen band. By varying the incidence angle, dispersion relations are revealed. Raman spectra calculations confirm the observation of one surface and two extraordinary guided polaritons and matches the evolution of PhPol frequency as a function of confinement. Resonant excitation close to the excitonic state singularly exalts the scattering efficiency of PhPol. Raman spectroscopy of PhPol in non-centrosymmetry 2D materials does not require any wavevector matching strategies. Widely available, it may accelerate the development of MIR nanophotonic devices and applications.

Published
2022
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2. Visible Out-of-plane Polarized Luminescence and Electronic Resonance from Black Phosphorus

Author

Schué, L., Goudreault, F. A., Righi, A., Resende, G. C., Lefebvre, V., Godbout, E., Pimenta, M. A., Côté, M., Francoeur, S., and Martel, R.

Subjects
Condensed Matter - Materials Science
Abstract

Black Phosphorus (BP) is unique among layered materials owing to its homonuclear lattice and strong structural anisotropy. While recent investigations on few layers BP have extensively explored the in-plane (a,c) anisotropy, much less attention has been given to the out-of-plane direction (b). Here, the optical response from bulk BP is probed using polarization-resolved photoluminescence (PL), photoluminescence excitation (PLE) and resonant Raman scattering along the zigzag, out-of-plane, and armchair directions. PL reveals an unexpected b-polarized emission occurring in the visible at 1.75 eV, far above the fundamental gap (0.3 eV). PLE indicates that this emission is generated through b-polarized excitation at 2.3 eV. The same electronic resonance is observed in resonant Raman scans, where the scattering efficiency of both Ag phonon modes is enhanced. These experimental results are fully consistent with DFT calculations of the permittivity tensor elements and demonstrate the remarkable extent to which the anisotropy influences the optical properties and carrier dynamics in black phosphorus.

Published
2021
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3. Taking advantage of light- and heavy-hole trions for optical spin initialization, control and readout

Author

Éthier-Majcher, G., St-Jean, P., and Francoeur, S.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Optical control strategies in semiconductor nanostructures have almost exclusively relied on heavy-hole exciton and trion states. In the first part of this letter, we show that light-hole trions provide the missing ressource for consolidating all single qubit operations in a mutually compatible magnetic field configuration: electron spin initialization and control can be achieved through light-hole trion states and cycling transition is provided by heavy-hole trion states. In the second part, we experimentally demonstrate that pairs of nitrogen atoms in GaAs exhibiting a Cs symmetry bind both light- and heavy-hole excitons and negative trions. A detailed analysis of the fine structure reveals that that trion states provide the lambda level structure necessary for fast initialization and control along with energetically-protected cycling transition compatible with single-shot readout., Comment: 4 figures

Published
2015

4. Exfoliating pristine black phosphorus down to the monolayer: photo-oxidation and electronic confinement effects

Author

Favron, A., Gaufrès, E., Fossard, F., Lévesque, P. L., Phaneuf-L'Heureux, A-L., Tang, N. Y-W., Loiseau, A., Leonelli, R., Francoeur, S., and Martel, R.

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Thin layers of black phosphorus have recently raised interest for their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of P atoms stacked together by weak van der Waals forces can be exfoliated down to the stratophosphane monolayer (also called phosphorene) using procedures similar to those used for graphene. Properties of this 2D material are however challenging to probe due to a fast and ubiquitous degradation upon exposure to ambient conditions. Herein, we investigate the crystal degradation using in-situ Raman and transmission electron spectroscopies and highlight a process involving a photo-induced oxidation reaction with adsorbed oxygen in water. The experimental conditions to prepare and preserve mono-, bi- and multilayers of stratophosphane in their pristine states were determined. Study on these 2D layers provides new insights on the effect of confinement on the chemical reactivity and the vibrational modes of black phosphorus., Comment: main text: 16 pages, 5 figures

Published
2014

17. Effects of symmetry-breaking perturbations on excitonic states bound to systems of reduced symmetry.

Author

Francoeur, S. and Marcet, S.

Subjects
*HAMILTONIAN systems, *CRYSTALS, *HOLES (Electron deficiencies), *PERTURBATION theory, *EIGENVALUES
Abstract

Using an invariant expansion, we build an Hamiltonian describing the influence of the crystal-field, the electron-hole exchange interaction, and any symmetry-breaking perturbations on the fine structure of excitons bound to systems of reduced symmetry: D2d, C3v, and C2v. Several perturbations are considered, including, but not limited to, an electric field, a magnetic field, a strain field, and their combinations. For each symmetry system considered, symmetrized excitonic wave functions, build from heavy- and light-holes states, are used to expand the Hamiltonian in the form of matrices, whose eigenvalues directly provide the energy of the excitonic states and whose eigenstates can be used to determine oscillator strengths of optical transitions. Using this model, we satisfactorily reproduce the excitonic emission observed from nitrogen dyads in GaAs and tellurium dyads in strained ZnSe. We also present Hamiltonians for independent heavy- and light-hole subsystems. Comparing the two models, we demonstrate that the splitting observed in strained quantum dots of C2v symmetry does not necessarily imply a significant anisotropic exchange interaction. This splitting can be produced by a weak coupling between heavy- and light-hole bands. [ABSTRACT FROM AUTHOR]

Published
2010
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18. Terrestrial runoff boosts phytoplankton in a Mediterranean coastal lagoon, but these effects do not propagate to higher trophic levels

Author

Liess, Antonia, Rowe, Owen, Francoeur, S. N., Guo, Junwen, Lange, K., Schroeder, A., Reichstein, Birte, Lefèbure, Robert, Deininger, Anne, Mathisen, Peter, Faithfull, Carolyn L., Liess, Antonia, Rowe, Owen, Francoeur, S. N., Guo, Junwen, Lange, K., Schroeder, A., Reichstein, Birte, Lefèbure, Robert, Deininger, Anne, Mathisen, Peter, and Faithfull, Carolyn L.

Abstract

Heavy rainfall events causing significant terrestrial runoff into coastal marine ecosystems are predicted to become more frequent with climate change in the Mediterranean. To simulate the effects of soil runoff on the pelagic food web of an oligotrophic Mediterranean coastal lagoon, we crossed soil extract addition (increasing nutrient availability and turbidity) and fish presence in a full factorial design to coastal mesocosms containing a natural pelagic community. Soil extract addition increased both bacteria and phytoplankton biomass. Diatoms however profited most from soil extract addition, especially in the absence of fish. In contrast zooplankton and fish did not profit from soil extract addition. Furthermore, our data indicate that nutrients (instead of light or carbon) limited basal production. Presumed changes in carbon availability are relatively unimportant to primary and secondary production in strongly nutrient limited systems like the Thau Lagoon. We conclude that in shallow Mediterranean coastal ecosystems, heavy rainfall events causing soil runoff will (1) increase the relative abundance of phytoplankton in relation to bacteria and zooplankton, especially in the absence of fish (2) not lead to higher biomass of zooplankton and fish, possibly due to the brevity of the phytoplankton bloom and the slow biomass response of higher trophic levels.

Published
2016
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20. Terrestrial runoff may reduce microbenthic net community productivity by increasing turbidity : a Mediterranean coastal lagoon mesocosm experiment

Author

Liess, Antonia, Faithfull, Carolyn, Reichstein, Birte, Rowe, Owen, Guo, Junwen, Pete, R., Thomsson, Gustaf, Uszko, Wojciech, Francoeur, S. N., Liess, Antonia, Faithfull, Carolyn, Reichstein, Birte, Rowe, Owen, Guo, Junwen, Pete, R., Thomsson, Gustaf, Uszko, Wojciech, and Francoeur, S. N.

Abstract

Terrestrial runoff into aquatic ecosystems may have both stimulatory and inhibitory effects, due to nutrient subsidies and increased light attenuation. To disentangle the effects of runoff on microbenthos, we added soil to coastal mesocosms and manipulated substrate depth. To test if fish interacted with runoff effects, we manipulated fish presence. Soil decreased microphytobenthic chlorophyll-a per area and per carbon (C) unit, increased microbenthic phosphorous (P), and reduced microbenthic nitrogen (N) content. Depth had a strong effect on the microbenthos, with shallow substrates exhibiting greater microbenthic net ecosystem production, gross primary production, and community respiration than deep substrates. Over time, micobenthic algae compensated for deeper substrate depth through increased chlorophyll-a synthesis, but despite algal shade compensation, the soil treatment still appeared to reduce the depth where microbenthos switched from net autotrophy to net heterotrophy. Fish interacted with soil in affecting microbenthic nutrient composition. Fish presence reduced microbenthic C/P ratios only in the no soil treatment, probably since soil nutrients masked the positive effects of fish excreta on microbenthos. Soil reduced microbenthic N/P ratios only in the absence of fish. Our study demonstrates the importance of light for the composition and productivity of microbenthos but finds little evidence for positive runoff subsidy effects.

Published
2015
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23. Phonon Engineering in Isotopically Disordered Silicon Nanowires

Author

Mukherjee, S., primary, Givan, U., additional, Senz, S., additional, Bergeron, A., additional, Francoeur, S., additional, de la Mata, M., additional, Arbiol, J., additional, Sekiguchi, T., additional, Itoh, K. M., additional, Isheim, D., additional, Seidman, D. N., additional, and Moutanabbir, O., additional

Published
2015
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27. Six new Actinella (Bacillariophyta) species from Papua New Guinea, Australia and New Zealand: further evidence for widespread diatom endemism, in the Australasian region

Author

Sabbe, K., Vanhoutte, K., Lowe, R.L., Bergey, E.A., Biggs, B.J.F., Francoeur, S., Hodgson, D., and Vyverman, W.

Subjects
Bacillariophyta [Diatoms], Actinella
Abstract

Examination of sediment samples from oligo-and dystrophic ponds, lakes and streams in Papua New Guinea. Australia and New Zealand revealed a hitherto unknown diversity of the diatom genus Actinella Lewis. Six new species are proposed, viz. Actinella aotearoaia sp. nov., A. giluwensis sp. nov., A. indistincta sp. nov., A. muylaertii sp, nov., A. parva sp. nov. and A. pulchella sp. nov. All species are heteropolar, both in girdle and valve view. Novel information on the genus Actinella includes the observations of two ribbon-shaped. valve-appressed plastids in A. aotearoaia and A. pulchella, and the presence of long mucilage stalks in A. aotearoaia. It is argued that, despite recent proposals to reduce the genera Actinella and Desmogonium Ehrenberg to the rank of subgenera of Eunotia, they should be kept separate until the taxonomic significance of their distinctive morphological features (such as heteropolarity) is fully assessed. The new species appear to be endemic to Australasia and have distinct biogeographies within this region. Except for A. aotearoaia, all species are present in Tasmania. A. indistincta and A. pulchella have also been found in New Zealand (Stewart Island). A. aotearoaia is common in several localities in New Zealand but was also found near Sydney on the Australian mainland. Actinella giluwensis has only been observed in material from Papua New Guinea. The record of A. punctata for the latter country constitutes the first confirmed record for this species outside North America and Europe. The discovery of the new Actinella species again confirms the importance of the Australasian region as a major centre of microalgal biodiversity and endemism.

Published
2001

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